The present invention concerns the manufacture of electrolytic capacitors, more particularly the lead frames for electrolytic capacitor anodes.
At present, electrolytic capacitors are generally made using an aluminium or tantalum anode and an electrolyte which may be solid or liquid.
A process of manufacture of capacitors with aluminium and with a solid electrolyte comprises several stages, a similar process being usable for capacitors with tantalum and a solid electrolyte. These stages are principally:
construction of the anode block either by compacting an aluminium wire, or by etching, folding or rolling of a sheet of aluminium, or by sintering; PA1 soldering of the anode block onto a tab of a lead frame; PA1 anodization of the aluminium block to form a thin layer of alumina (oxidation); PA1 formation of the solid electrolyte (impregnation and pyrolysis); PA1 formation of the cathode (graphitization and/or silvering); and PA1 molding or potting.
The manufacturing process of these capacitors is rather complicated and the problems presently encountered during the manufacture of electrolytic capacitors are of several types.
The connection of the anode block to the tab of the lead frame, which is performed by soldering, is subject to dimensional constraints to be respected to take into account the different phases of manufacture.
To deposit the electrolyte and then the graphite, the anodized blocks are soaked in different solutions which have good wettability, and it can be observed that the conducting electrolyte and graphite rise by capillary action around the solder. There is therefore a risk of short-circuiting between the non-oxidized anode and the cathode constituted by the electrolyte. This rise by capillarity is not controllable and varies depending on the materials and their surface states.
To prevent such short-circuiting, dimensions are imposed: a minimum height for the oxidation of the tab of the lead frame and a lower minimum height for electrolytic impregnation and graphitization, taking into account the capillary rise. There is therefore an oxidized part of the anode which is not impregnated and which is inactive from a capacitive point of view.